Myocardial infarction (MI) is extremely common worldwide. It occurs following the rupture of small, usually thin-walled lipid-filled plaques in the coronary arteries. When such plaques rupture, they release thrombogenic factors into the blood, causing a cascading sequence of events that culminates with clot formation and blood vessel occlusion. The plaques that obstruct a relatively small percentage of the lumen (for example, about 15% or so) have been undetectable by conventional imaging modalities (due to, primarily, insufficient resolution). The commercialization of the OCT systems and methods alleviated this situation to some degree. However, although the optical coherence tomography (OCT) imaging modality was shown to identify small plaques and their thin intimate, thin-walled caps (about 75 microns in diameter or less), when used for intravascular imaging, it is well recognized that the ability of the OCT to distinguish lipid plaque material (which is intravascularly unstable) from non-lipid plaque material (which has higher stability) is quite poor. Indeed, both the lipid and non-lipid appear similar (for example, dark) when imaged with an OCT system. As a result, interventional treatments cannot be performed on the otherwise visible thin-capped intravascular plaques until the lipid nature of these plaques if identified reliably.
Overcoming this obstacle is essential for therapeutic treatment related to plaques that cause most of the MIs. It has been proposed that in an OCT-image of a plaque, a diffuse (non-sharp) border between the intima and the core of the plaque may be indicative of the presence of lipid material. However, our prior work shows that such diffuse boundary could be explained by surface scattering in the intima of the plaque. (See, for example, Brezinski M. E. et al, Assessing atherosclerotic plaque morphology: Comparison of optical coherence tomography and high frequency intravascular ultrasound. Heart. 77:397-403, 1997, PMID: 9196405). FIG. 7 is a pictorial representation of an OCT image of a plaque, where the diffuse cap-lipid interface (traced with a dashed line 710 and pointed out with an arrow 712) is covered with a highly-scattering cap (seen in the upper “reflectogram” portion of FIG. 7 as an area 716 producing significant reflections). Arrows 718 identify a cap with lower scattering and the cap-lipid interface in this area is sharply defined. Arrows 720 indicate the intima-elastic layer interface (no lipid preset) that is diffuse, with an intima that is highly scattering. The abovementioned surface-scattering explanation of the diffuse boundary appearing in the OCT images was confirmed by independent studies that showed the identification of lipid plaques based on the diffuse border in an image with insufficient sensitivity and/or reliability of 83% and as low as 43%, for example.
There remains a persisting need, therefore, in developing a technique that would allow the reliable and repeatable lipid-plaque identification and the ability to unambiguously distinguish a lipid plaque from a non-lipid plaque. Embodiments of the present invention address such need by employing aspects of the OCT-based approach that have been previously neglected in related art.